Non-oriented electrical steel sheet and method for manufacturing the same

ABSTRACT

The present invention provides a non-oriented electrical steel sheet at low cost that has excellent magnetic properties and mechanical properties as well as excellent quality of steel sheet. The non-oriented electrical steel sheet has a chemical composition containing, by mass %, Si: 5.0% or less, Mn: 2.0% or less, Al: 2.0% or less, and P: 0.05% or less, in a range satisfying formula (1), and furthermore, C: 0.008% or more and 0.040% or less; N: 0.003% or less, and Ti: 0.04% or less, in a range satisfying formula (2), with the balance composed of Fe and incidental impurities: 
       300≦85[Si %]+16[Mn %]+40[Al %]+490[P %]≦430  (1)
 
       0.008≦Ti*&lt;1.2[C %]  (2),
 
     where Ti*=Ti−3.4[N %].

CROSS REFERENCE TO RELATED APPLICATION

This is the U.S. National Phase application of PCT/JP2011/001074, filedFeb. 24, 2011, the disclosure of which is incorporated herein byreference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a non-oriented electrical steel sheet,and in particular, to a non-oriented electrical steel sheet having highstrength and excellent fatigue properties, and furthermore, excellentmagnetic properties that is suitably used for components that aresubject to high stress, typically, drive motors for turbine generators,electric vehicles and hybrid vehicles, or rotors for high-speed rotatingmachinery, such as servo motors for robots, machine tools or the like,and a method for manufacturing the same. Additionally, the presentinvention provides the above-described non-oriented electrical steelsheet at low cost as compared to the conventional art.

BACKGROUND OF THE INVENTION

As recent, advances in motor drive systems have enabled frequencycontrol of drive power sources, more and more motors are offeringvariable-speed operation and enabling high-speed rotation at frequencieshigher than the commercial frequency. In such motors enabling high-speedrotation, the centrifugal force acting on a rotating body isproportional to the radius of rotation and increases in proportion tothe square of the rotational speed. Accordingly, in particular, rotormaterials for middle- and large-sized high speed motors require highstrength.

In addition, in IPM (interior permanent magnet)-type DC inverter controlmotors, which have been increasingly employed for motors in hybridvehicles, such as drive motors or compressor motors, stress isconcentrated on portions between grooves for embedding magnets in arotor and the outer circumference of the rotor, or at narrow bridgeportions of several millimeters width between the grooves for embeddingmagnets. Since motors can be reduced in size with increasing rotationalspeed, there is a growing demand for increasing the rotational speed ofmotors, such as in drive motors for hybrid vehicles with space andweight constraints. As such, high strength materials are advantageouslyused as core materials for use in rotors of high speed motors.

On the other hand, since rotating equipment such as motors or generatorsmakes use of electromagnetic phenomenon, the core materials of ironcores of rotating equipment are also required to have excellent magneticproperties. In particular, it is necessary for rotors of high speedmotors to assume low iron loss at high frequency; iron loss at highfrequency would otherwise lead to a rise in core temperature due to theeddy current induced by a high-frequency magnetic flux, causing thermaldemagnetization of embedded permanent magnets, reducing motorefficiency, and so on. Therefore, there is a demand for such anelectrical steel sheet as a material for rotors that possesses highstrength and excellent magnetic properties.

Steel-strengthening mechanisms include solid solution strengthening,precipitation strengthening, crystal grain refinement, work hardening,and so on.

To date, a number of high-strength non-oriented electrical steel sheetshave been considered and proposed to meet the needs, such as those ofrotors of high speed motors.

As an example utilizing solid solution strengthening, for instance, JP60-238421 A (PTL 1) proposes a method for increasing the strength ofsteel by adding elements such as Ti, W, Mo, Mn, Ni, Co or Al to thesteel for the purposes of primarily increasing Si content from 3.5% to7.0%, and furthermore, achieving solid solution strengthening. Moreover,in addition to the above-described strengthening methods, JP 62-112723 A(PTL 2) proposes a method for improving magnetic properties bycontrolling the crystal grain size in the range of 0.01 mm to 5.0 mmthrough manipulation of the final annealing conditions.

However, when these methods are applied to factory production, thefactory production may be more prone to a trouble such as sheet fracturein a rolling line after hot rolling, which would cause a reduction inyield and production line stop by necessity. Sheet fracture may bereduced if cold rolling is performed in warm conditions at sheettemperatures of hundreds of degrees centigrade, in which case, however,process control issues will be of considerable concern, such asadaptation of the facility to warm rolling, tighter productionconstraints, and so on.

In addition, as a technique utilizing precipitation of carbonitrides, JP06-330255 A (PTL 3) proposes a technique that makes use of strengtheningby precipitation and grain refining effects provided by carbonitrides insteel, the steel containing Si in the range of 2.0% or more and lessthan 4.0%, C in the range of 0.05% or less, and one or two of Nb, Zr, Tiand V in the range of 0.1<(Nb+Zr)/8(C+N)<1.0, and 0.4<(Ti+V)/4(C+N)<4.0.Similarly, JP 02-008346 A (PTL 4) proposes a technique, in addition tothe features described in PTL 3, to add Ni and Mn in a total amount of0.3% or more and 10% or less to steel for solid solution strengthening,and further add Nb, Zr, Ti and V in the same ratios as those describedin PTL 3 to the steel, thereby balancing high strength with magneticproperties.

However, if these methods are applied to obtain high strength, problemsarise that not only unavoidably cause a deterioration of magneticproperties, but also make the resulting products susceptible to surfacedefects such as scabs caused by precipitates, internal defects, and soon, resulting in lower product quality, and furthermore, prone to areduction in yield due to removal of defects and a fracture troubleduring production of steel sheets, resulting in an increased cost. Inaddition, the technique described in PTL 4 will lead to an even greaterincrease in cost because it involves adding an expensivesolid-solution-strengthening element, such as Ni.

Further, as a technique utilizing work hardening, JP 2005-113185 A (PTL5) proposes a technique for enhancing the strength of steel containingSi in the range of 0.2% to 3.5% by allowing worked microstructures toremain in the steel material. Specifically, PTL 5 discloses means thatdoes not perform heat treatment after cold rolling, or, if it does,retains the steel material at 750° C. for 30 seconds at most, preferablyat 700° C. or lower, more preferably at 650° C. or lower, 600° C. orlower, 550° C. or lower, and 500° C. or lower. PTL 5 reports the actualresults indicating that the worked microstructure ratio is 5% withannealing at 750° C. for 30 seconds, 20% with annealing at 700° C. for30 seconds, and 50% with annealing at 600° C. for 30 seconds. In thiscase, there is a problem that such low annealing temperatures lead toinsufficient shape correction of rolling strips. Improperly-shaped steelsheets have a problem that would lead to a lower stacking factor afterworked into a motor core in a stacked fashion, a non-uniform stressdistribution when rotating at high speed as a rotor, and so on. There isanother problem that the ratio of worked grains to recrystallized grainsvaries greatly with the steel compositions and annealing temperatures,which makes it difficult to obtain stable properties. Further, anon-oriented electrical steel sheet is generally subjected to finalannealing using a continuous annealing furnace, which is usuallymaintained in an atmosphere containing at least several percent ofhydrogen gas in order to reduce oxidation of surfaces of the steelsheet. To carry out low-temperature annealing at temperatures below 700°C. in such a continuous annealing facility, there will be tremendousoperational constraints, such as requirements of time-consumingswitching of furnace temperature settings, replacement of the atmospherein the furnace for avoiding hydrogen explosion, and so on.

In view of the aforementioned technical background, the inventors of thepresent invention proposed in JP 2007-186790 A (PTL 6) a high strengthelectrical steel sheet balancing the ability of shape correction of thesteel sheet with the ability of strengthening by non-recrystallizedmicrostructures during final annealing, which steel sheet is obtained byadding Ti sufficiently and excessively in relation to C and N to asilicon steel with reduced C and N contents and thereby raising therecrystallization temperature of the silicon steel. This method stillhas a difficulty in that it may increase alloy cost due to a relativelyhigh Ti content, cause variations in mechanical properties due to theremaining recrystallized microstructures, and so on.

PATENT LITERATURE

-   PTL 1: JP 60-238421 A-   PTL 2: JP 62-112723 A-   PTL 3: JP 6-330255 A-   PTL 4: JP 2-008346 A-   PTL 5: JP 2005-113185 A-   PTL 6: JP 2007-186790 A

SUMMARY OF THE INVENTION

As described above, some proposals have been made on high-strengthnon-oriented electrical steel sheets. In the proposals made to date,however, it has not been possible until now to manufacture, with the useof an ordinary facility for manufacturing electrical steel sheets, sucha high-strength non-oriented electrical steel sheet in an industriallystable manner with good yield and at low cost that has good magneticproperties in addition to high tensile strength and high fatiguestrength, and furthermore, satisfy the quality requirements of steelsheet, such as those relating to surface defects, internal defects,sheet shape or the like. Particularly, the high-strength electricalsteel sheets that have so far been provided for rotors of high speedmotors are in a situation where the resulting rotors will be subject tounavoidable heat generation due to their magnetic property, i.e., highiron loss at high frequency, which necessarily poses limitations on thedesign specification of the motors.

Therefore, the present invention aims to provide a high-strengthnon-oriented electrical steel sheet at low cost, having excellentmagnetic properties and quality of steel sheet, and a method formanufacturing the same. Specifically, the present invention aims toprovide means for manufacturing such a non-oriented electrical steelsheet in an industrially stable manner and yet at low cost that has botha tensile strength of 650 MPa or more, desirably 700 MPa or more, andgood low iron loss properties at high frequency such that, for example,a steel material having a sheet thickness of 0.35 mm has a value ofW_(10/400) of 40 W/kg or lower, desirably 35 W/kg or lower.

The inventors of the present invention made intensive studies onhigh-strength electrical steel sheets that can achieve theabove-described objects at a high level and methods for manufacturingthe same. As a result, the inventors have discovered that the amount andratio of Ti and C to be added to steel are deeply concerned with thebalance between the strength properties and the magnetic properties ofan electrical steel sheet, and that a high-strength electrical steelsheet having excellent properties may be manufactured in an stablemanner and at low cost by optimizing the amount of precipitation of Ticarbides.

That is, the present invention relies upon the following findings:

-   (A) The growth of crystal grains of an electrical steel sheet during    final annealing may be inhibited by the presence of a relatively    small amount of Ti carbides, whereby strengthening by refinement of    crystal grains may be achieved.-   (B) The presence of excessive Ti carbides does not contribute to    effective inhibition of the growth of crystal grains, but rather has    adverse effects such as causing more surface defects and internal    defects, degrading quality of steel sheet, contributing to origins    of fracture, and so on. To this extent, surface defects such as    scabs and internal defects are significantly reduced by controlling    the amount of Ti to be added to the steel within an appropriate    range.    -   On the other hand, Ti nitrides are formed at higher temperatures        than Ti carbides. Thus, they are less effective for inhibiting        the growth of crystal grains and not useful for crystal grain        refinement control intended by the present invention. Therefore,        in an approach for inhibiting the growth of crystal grains by        controlling the amount of Ti carbides, it is desirable to reduce        the N content in a stable manner. This is entirely different        from the conventional approaches utilizing strengthening by        precipitation, where the effects of C and N are dealt with in        the same manner.-   (C) In a steel sheet with refined crystal grains, solute C has an    effect of not only enhancing tensile strength, but also improving    fatigue properties essentially required for a rotor material    rotating at high speed.-   (D) Major alloy components that are normally added for the purpose    of reducing iron loss by increasing the electrical resistance of an    electrical steel sheet are Si, Al and Mn. These three substitutional    alloy elements also have an effect of implementing solid solution    strengthening of steel. Accordingly, the balance between high    strength and low iron loss is effectively ensured on the basis of    the solid solution strengthening by these elements. However, there    is a limit in adding these elements since excessive addition leads    to embrittlement of steel and poses difficulty in manufacturing    steel. Si-based addition is desirable for satisfying the    requirements of solid solution strengthening, lower iron loss and    productivity in most efficient way.

Based on these findings, the inventors of the present invention foundthat a properly balanced utilization of solid solution strengtheningwith the use of the substitutional alloy elements mainly composed of Si,crystal grain refinement with Ti carbides, and solid solutionstrengthening with an interstitial element of C may provide anon-oriented electrical steel sheet that has high strength, excellentfatigue properties under the conditions of use, and furthermore,excellent magnetic properties and quality of steel sheet, withoutsubstantially adding extra constraints on manufacture of steel sheets oradditional steps to the normal production of non-oriented electricalsteel sheets, and also found a method necessary for manufacturing thesame. As a result, the inventors accomplished the present invention.

That is, the primary features of exemplary embodiments of the presentinvention are as follows.

(i) A non-oriented electrical steel sheet comprising, by mass %:

-   -   Si: 5.0% or less;    -   Mn: 2.0% or less;    -   Al: 2.0% or less; and    -   P: 0.05% or less,        in a range satisfying formula (1), and the steel sheet further        comprising, by mass %:    -   C: 0.008% or more and 0.040% or less;    -   N: 0.003% or less; and    -   Ti: 0.04% or less,        in a range satisfying formula (2), the balance being composed of        Fe and incidental impurities:

300≦85[Si %]+16[Mn %]+40[Al %]+490[P %]≦43.0  (1)

0.008≦Ti*<1.2[C %]  (2)

-   -   where        -   Ti*=Ti−3.4[N %], and        -   the [Si %], [Mn %], [Al %], [P %], [C %] and [N %] represent            the contents (mass %) of the indicated elements,            respectively.

(ii) The non-oriented electrical steel sheet according to (i) above,wherein the Si, Mn, Al and P contents are, by mass %,

-   -   Si: more than 3.5% but not more than 5.0%,    -   Mn: 0.3% or less,    -   Al: 0.1% or less, and    -   P: 0.05% or less.

(iii) The non-oriented electrical steel sheet according to (i) or (ii)above, further comprising, by mass %, at least one of:

-   -   Sb: 0.0005% or more and 0.1% or less;    -   Sn: 0.0005% or more and 0.1% or less;    -   B: 0.0005% or more and 0.01% or less;    -   Ca: 0.001% or more and 0.01% or less;    -   REM: 0.001% or more and 0.01% or less;    -   Co: 0.05% or more and 5% or less;    -   Ni: 0.05% or more and 5% or less; and    -   Cu: 0.2% or more and 4% or less.

(iv) A method for manufacturing a non-oriented electrical steel sheet,comprising:

-   -   subjecting a steel slab to soaking, where the steel slab is        retained at a soaking temperature of 1000° C. to 1200° C., the        steel slab containing, by mass %,    -   Si: 5.0% or less,    -   Mn: 2.0% or less,    -   Al: 2.0% or less, and    -   P: 0.05% or less,        in a range satisfying formula (1), and the steel slab further        containing, by mass %,    -   C: 0.008% or more and 0.040% or less,    -   N: 0.003% or less, and    -   Ti: 0.04% or less,        in a range satisfying formula (2);    -   subjecting the steel slab to subsequent hot rolling to obtain a        hot-rolled steel material;    -   then subjecting the steel material to cold rolling or warm        rolling once, or twice or more with intermediate annealing        performed therebetween, to be finished to a final sheet        thickness; and    -   subjecting the steel material to final annealing, wherein prior        to the final annealing, the steel material is subjected to heat        treatment at least once where the steel material is retained at        temperatures of 800° C. or higher and 950° C. or lower for 30        seconds or more, and subsequently to the final annealing at        700° C. or higher and 850° C. or lower:

300≦85[Si %]+16[Mn %]+40[Al %]+490[P %]≦430  (1)

0.008≦Ti*<1.2[C %]  (2),

-   -   where Ti*=Ti−3.4[N %].

(v) The method for manufacturing a non-oriented electrical steel sheetaccording to (iv) above, wherein the Si, Mn, Al and P contents are, bymass %,

-   -   Si: more than 3.5% but not more than 5.0%,    -   Mn: 0.3% or less,    -   Al: 0.1% or less, and    -   P: 0.05% or less.

(vi) The method for manufacturing a non-oriented electrical steel sheetaccording to (iv) or (v) above, wherein the steel slab further contains,by mass %, at least one of;

-   -   Sb: 0.0005% or more and 0.1% or less;    -   Sn: 0.0005% or more and 0.1% or less;    -   B: 0.0005% or more and 0.01% or less;    -   Ca: 0.001% or more and 0.01% or less;    -   REM: 0.001% or more and 0.01% or less;    -   Co: 0.05% or more and 5% or less;    -   Ni: 0.05% or more and 5% or less; and    -   Cu: 0.2% or more and 4% or less.

According to the present invention, a non-oriented electrical steelsheet may be provided that is excellent in both mechanical propertiesand magnetic properties required for a rotor material of motors rotatingat high speed, and that has excellent quality of steel sheet in terms ofscab, sheet shape, and so on. The present invention also allows stableproduction of such non-oriented electrical steel sheets with high yield,without incurring a significant increase in cost or imposing severeconstraints on manufacture or requiring extra steps, as compared to thenormal production of non-oriented electrical steel sheets. Therefore,the present invention is applicable in the field of motors, such asdrive motors of electric vehicles and hybrid vehicles or servo motors ofrobots and machine tools, where demand for higher rotational speed isexpected to grow in the future. Thus, the present invention has a highindustrial value and makes a significant contribution to the industry.

BRIEF DESCRIPTION OF THE DRAWING

The present invention according to exemplary embodiments will be furtherdescribed below with reference to the accompanying drawings, wherein:

FIG. 1 is a graph illustrating the relationship between Ti content andtensile strength;

FIG. 2 is a graph illustrating the relationship between Ti content andiron loss; and

FIG. 3 is a graph illustrating the relationship between Ti content andsurface scab defect rate.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The experimental results underlying the present invention will bedescribed in detail below.

That is, the inventors of the present invention investigated in detailhow Ti, which is a major carbonitride forming element, affects thequality of steel sheet in terms of strengthening by precipitation,recrystallization, grain growth behavior, scabs, and so on. As a result,it was found that Ti has significantly different effects, in particular,when added so that the resulting Ti content is equal to or less than atotal content of C and N in atomic fraction, and has an optimum range ofaddition for satisfying the requirements at a high level regarding highstrength as well as magnetic properties and quality of steel sheet. Themajor experimental results will be described below. The percentage “%”of each steel component represents “mass %,” unless otherwise specified.

<Experiment 1>

Steel samples, which have steel compositions mainly composed of silicon(Si): 4.0% to 4.1%, manganese (Mn): 0.03% to 0.05%, aluminum (Al):0.001% or less, phosphorus (P): 0.007% to 0.009%, and sulfur (S): 0.001%to 0.002%, containing substantially constant amounts of carbon (C):0.024% to 0.026% and nitrogen (N): 0.001% to 0.002%, and differentamounts of titanium (Ti) in the range of 0.001% to 0.36%, were obtainedby steelmaking in a vacuum melting furnace. These steel samples wereheated to 1100° C. and then subjected to hot rolling to be finished to athickness of 2.1 mm, respectively. Then, the steel samples weresubjected to hot band annealing at 900° C. for 90 seconds and further tocold rolling to be finished to a thickness of 0.35 mm, after which theoccurrence of scab defects on the surfaces of the steel sheets (scabsize per unit area) was determined. Subsequently, the steel samples weresubjected to final annealing at 800° C. for 30 seconds and evaluated fortheir mechanical properties (by using JIS No. 5 tensile test specimenscut parallel to the rolling direction) and magnetic properties (by usingEpstein test specimens cut in the rolling direction and transversedirection, measuring iron loss W_(10/400) with a magnetizing fluxdensity of 1.0 T and frequency of 400 Hz). The research results oftensile strength, magnetic property and occurrence of surface scabdefect are depicted in FIGS. 1, 2 and 3 as a function of Ti content,respectively.

Firstly, as illustrated in FIG. 1, tensile strength increases withaddition of Ti. However, it was found that this effect is lesspronounced within a Ti content range indicated by “A” (Range A) in FIG.1 where Ti content is smaller, while stable improvements in strength areobserved within a Ti content range indicated by “B” (Range B) in thefigure. Additionally, even further improvements in strength are achievedwithin a range indicated by “C” (Range C) in the figure where Ti contentis higher. Upon observation of steel structure in these regions, it wasfound that in Range B, the steel structure contains homogeneousmicrostructures with a crystal grain size of 10 μM or less, whereas inRange A, it involves crystal grains grown more than in Range B,particularly, mixed-grain-size microstructures with partial graingrowth. On the other hand, in Range C, the steel structure assumes amulti-phase of non-recrystallized grains and recrystallized grains.

FIG. 2 illustrates the relationship between Ti content and iron lossW_(10/400). While good iron loss properties are obtained in Range A withthe lowest iron loss, as illustrated in FIG. 1, Range A shows lowerstrength levels. On the other hand, while high strength materials areobtained in Range C and D in FIG. 2, iron loss is also high in theseranges. In contrast, Range B offers materials that have iron lossproperties almost as good as in Range A, while yielding strength resultscomparable to those obtained in Range C.

On the other hand, as illustrated in FIG. 3, the scab defect rate startsto increase when Ti content exceeds 0.04%, and continues to rise up toaround a point at which the equivalent ratio of elements of Ti to C andN is equal to 1, where a substantially constant rate of scab generationis reached. Assuming constant C and N contents, the amount of Ticarbonitride precipitates continues to increase up to around a point atwhich this equivalent ratio of elements is equal to 1, and then remainsconstant. Thus, it is considered that the amount of Ti carbonitrideprecipitates is related to the amount of scab generation. These resultsrevealed that by controlling Ti content within range B, it becomespossible to balance high strength and low iron loss, while reducing scabdefects that would otherwise cause a reduction in yield and a sheetfracture trouble and be directly linked to an increase in manufacturingcost. That is, it is advantageous to contain Ti in an amount of 0.04% orless in terms of reducing scab defects, provided that it is sufficientfor forming a certain amount of Ti carbonitrides.

In addition, as a result of further studies conducted with the samecomponents except for the above-described steel and N content and withvarying N contents, it was also found that the lower limit of Ti contentto which high strength can be obtained increases with increasing Ncontents. Still further studies revealed that it is necessary to satisfya relation of 0.008≦Ti* (where Ti*=Ti−3.4[N %]). From this, it isbelieved that since Ti carbides make a large contribution to enhancementof strength while Ti nitrides contribute less, control of Ti carbides ismore important.

These results revealed that by controlling Ti content at a level ofRange B, it becomes possible to balance high strength and low iron loss,while reducing scab defects that would otherwise cause a reduction inyield and a sheet fracture trouble and be directly linked to an increasein manufacturing cost.

<Experiment 2>

Then, to investigate details of the influence of Ti carbonitrides, steelsamples having compositions shown in Table 1 were prepared bysteelmaking in a vacuum melting furnace to obtain steel sheets, eachhaving a sheet thickness of 0.35 mm, following the same procedure as inExperiment 1. C and N contents of steel samples were varied using steelsample “a,” which has small C and N contents, as a reference. Steelsamples “c” and “d” contain C and N so that the total content thereof iswithin a predetermined range. The surface scab defect rate, iron lossand tensile strength of the resulting samples are shown in Table 2.While steel samples “b,” “c” and “d” show an increase in strength inrelation to steel sample “a,” comparing steel samples “c” and “d” havingsubstantially the same total amount of C and N to evaluate the effect ofaddition of C and N, it can be seen that steel sample “c” having a lowerN content has higher strength. Upon observation of microstructures, itwas found that the steel samples are listed as a>d>b>c in descendingorder of crystal grain size, as is the case with in descending order oftensile strength.

TABLE 1 (mass %) Steel Si Mn Al P C N Ti a 4.33 0.07 0.0005 0.010 0.00190.0021 0.0302 b 4.32 0.05 0.0010 0.010 0.0240 0.0009 0.0295 c 4.29 0.030.0007 0.010 0.0293 0.0009 0.0298 d 4.25 0.08 0.0018 0.020 0.0249 0.00520.0301

TABLE 2 Tensile Fatigue Limit Strength Surface Scab W_(10/400) StrengthStrength Ratio Defect Rate Steel (W/kg) TS (MPa) FS (MPa) FS/TS (m/m²) a26.9 641 535 0.83 0.000 b 33.0 722 630 0.87 0.003 c 32.5 730 665 0.910.003 d 31.0 676 540 0.80 0.004

These samples were further investigated for their fatigue properties.Tests were conducted in a tension-to-tension mode with a stress ratio of0.1 at a frequency of 20 Hz, where the fatigue limit strength is definedas a stress which allows a sample to survive 10 million stress amplitudecycles. The results thereof are also shown in Table 2. While a tendencyis observed that materials having a higher tensile strength TS possess ahigher fatigue limit strength FS, the strength ratio FS/TS differs fordifferent materials. In this case, steel sample “c” gave the bestresult. On the other hand, steel sample “d” does not improve so much infatigue limit strength for its high tensile strength. Given thesecircumstances, and as a result of our detailed investigations of themicrostructures of steel sample “d,” many precipitates, presumably TiNprecipitates having a grain size of greater than 5 μm were scatteredover the microstructures, and these precipitates were estimated ascontributing to origins of fatigue fracture. It should be noted herethat nitrogen reacts with titanium at relatively high temperatures of1100° C. or higher and tends to precipitate as TiN coarsely. It was thusbelieved that TiN tends to provide origins of fatigue fracture and isless effective as compared to Ti carbides for inhibiting the growth ofcrystal grains, which is one of the goals of the present invention.

On the other hand, when comparing steel samples “b” and “c,” it was alsofound that steel sample “c” gives better results in terms of tensilestrength and fatigue limit strength, and is particularly characterizedby its relatively high fatigue limit strength and high strength ratioFS/TS. Since steel samples “b” and “c” have substantially the same Tiand N contents, they exhibit similar precipitation behavior of Tinitrides and Ti carbides. It is thus believed that the differencebetween them is attributed to the difference in the amount of solutecarbon. Accordingly, it is estimated that the presence of solute carbonreduced the occurrence and propagation of cracks and increased fatiguelimit strength by locking dislocations introduced during repeated stresscycles such as found in fatigue test. Therefore, it is also important toensure formation of solute carbon.

Based on the above-described experimental results, the inventors of thepresent invention made further studies on how these factors including Ticarbides, Ti nitrides and solute carbon, with the addition of arelatively small amount of Ti, affect the steel structure, quality ofsteel sheet surface, as well as mechanical properties and magneticproperties of steel sheets. As a result, the inventors discovered therules comprehensively applicable to these factors and accomplished thepresent invention.

The present invention according to exemplary embodiments will now bedescribed in detail below.

Firstly, the grounds for the limitations with regard to the major steelcomponents are described.Steel of an embodiment of the present invention contains Si: 5.0% orless, Mn: 2.0% or less, Al: 2.0% or less, and P: 0.05% or less in arange satisfying formula (1):

300≦85[Si %]+16[Mn %]+40[Al %]+490[P %]≦430  (1)

The present invention aims to provide an electrical steel sheet havinghigh strength and excellent magnetic properties at low cost. To thisend, it is necessary to achieve solid solution strengthening above acertain level by means of the above-described four major alloycomponents. Thus, it is important to specify the contents of the fourmajor alloy components as described later, and to add these componentsto the steel so that the total amount of these alloy components iswithin a range satisfying the above formula (1), considering individualcontributions to solid solution strengthening. That is, if formula (1)gives a result less than 300, the strength of the resulting material isinsufficient, whereas if formula (1) gives a result more than 430, thereare more troubles with sheet cracking at the time of manufacture ofsteel sheets, leading to a deterioration in productivity and asignificant increase in manufacturing cost.

Next, the grounds for the limitations on the individual contents of thefour major alloy components are described.

Si≦5.0%

Silicon (Si) is generally used as a deoxidizer and one of the majorelements that are contained in a non-oriented electrical steel sheet andhave an effect of increasing the electrical resistance of steel toreduce its iron loss. Further, Si has high solid solution strengtheningability. That is, Si is an element that is positively added to thenon-oriented electrical steel sheet because it is capable of achievinghigher tensile strength, higher fatigue strength and lower iron loss atthe same time in a most balanced manner as compared to othersolid-solution-strengthening elements, such as Mn, Al or Ni, that areadded to the non-oriented electrical steel sheet. To this end, it isadvantageous to contain Si in steel in an amount of 3.0% or more, morepreferably exceeding 3.5%. However, above 5.0%, toughness degradationwill be pronounced, which should necessitate highly-sophisticatedcontrol during sheet passage and rolling processes, resulting in lowerproductivity. Therefore, the upper limit of the Si content is to be 5.0%or less.

Mn≦2.0%

Manganese (Mn) is effective in improving hot shortness properties, andalso has effects of increasing the electrical resistance of steel toreduce its iron loss and enhancing the strength of steel by solidsolution strengthening. Thus, Mn is preferably contained in steel in anamount of 0.01% or more. However, addition of Mn is less effective inimproving the strength of steel as compared to Si and excessive additionthereof leads to embrittlement of the resulting steel. Therefore, the Mncontent is to be 2.0% or less.

Al≦2.0%

Aluminum (Al) is an element that is generally used in steel refining asa strong deoxidizer. Further, as is the case with Si and Mn, Al also haseffects of increasing the electrical resistance of steel to reduce itsiron loss and enhancing the strength of steel by solid solutionstrengthening. Therefore, Al is preferably contained in steel in anamount of 0.0001% or more. However, addition of Al is less effective inimproving the strength of steel as compared to Si and excessive additionthereof leads to embrittlement of the resulting steel. Therefore, the Alcontent is to be 2.0% or less.

P≦0.05%

Phosphorus (P) is extremely effective in enhancing the strength of steelbecause it offers a significantly high solid solution strengtheningability even when added in relatively small amounts. Thus, P ispreferably contained in steel in an amount of 0.005% or more. However,excessive addition of P leads to embrittlement of steel due tosegregation, causing intergranular cracking or a reduction inrollability. Therefore, the P content is limited to 0.05% or less.

Additionally, among these major alloy elements Si, Mn, Al and P, aSi-based alloy design is advantageous for balancing solid solutionstrengthening/low iron loss and productivity in a most efficient way.That is, it is advantageous to contain Si in steel in an amount of morethan 3.5% for optimizing the balance of properties of the non-orientedelectrical steel sheet, where the contents of the remaining threeelements are preferably controlled as follows: Mn: 0.3% or less, Al:0.1% or less, and P: 0.05% or less. The grounds for the limitations onthe upper limit are as described above.

In addition, C, N and Ti are also important elements in the presentinvention. This is because it is important to inhibit the growth ofcrystal grains during steel sheet annealing with the use of a properamount of fine Ti carbides and to develop an ability of reinforcingcrystal grain refinement. For this purpose, it is necessary to containC: 0.008% or more and 0.040% or less, N: 0.003% or less, and Ti: 0.04%or less in steel, in a range satisfying formula (2):

0.008≦Ti*<1.2[C %]  (2)

where Ti*=Ti−3.4[N %].

0.008%≦C≦0.040%

Carbon (C) needs to be contained in steel in an amount of 0.008% ormore. That is, a carbon content of less than 0.008% makes it difficultto provide stable precipitation of fine Ti carbides and results in aninsufficient amount of solute C, in which case a further improvement infatigue strength is no longer possible. On the other hand, excessiveaddition of C leads to a deterioration in magnetic properties, whilebecoming a factor responsible for an increase in cost, such as makingwork hardening more pronounced during cold rolling and causing sheetfracture, forcing more rolling cycles due to an increased rolling load,and so on. Therefore, the upper limit of C is limited to 0.04%.

N≦0.003%

Nitrogen (N) forms nitrides with Ti, which are, however, formed athigher temperatures than Ti carbides. Thus, N is less effective ininhibiting the growth of crystal grains and not effective so much inrefining crystal grains. Rather, N sometimes causes adverse effects suchas providing origins of fatigue fracture. Therefore, N content islimited to 0.003% or less. Additionally, without limitation, the lowerlimit is preferably about 0.0005% in terms of steelmaking degassingability and for avoiding a deterioration in productivity due to a longrefining duration.

Ti≦0.04%

Control of titanium (Ti) carbides is advantageous in the presentinvention. Ti tends to form nitrides rather than carbides at hightemperatures. Thus, it is necessary to control the amount of Ti formingcarbides. If the amount of Ti that is capable of forming carbides isdenoted as Ti*, Ti* is represented as the Ti content minus the atomequivalent with N, namely:

Ti*=Ti−3.4[N %]

To allow the added Ti to precipitate as Ti carbides for enhancing thestrength of steel, while inhibiting the growth of crystal grains forpreventing an increase in iron loss of the steel, it is necessary to usea proper amount of C and satisfy Ti*≧0.008. On the other hand, if Ticontent is increased in relation to C content, there is a reduction inthe amount of solute C, in which case a further improvement in fatiguestrength is no longer possible. Therefore, it is also necessary tosatisfy Ti*<1.2[C %] at the same time.

In addition, if Ti content exceeds 0.04%, as previously described withreference to FIG. 3, more scab defects will occur and the quality ofsteel sheet and yield will be reduced, resulting in an increase in cost.Therefore, the upper limit of Ti content is to be 0.04%.

The present invention may also contain elements other than theaforementioned elements without impairing the effects of the invention.For example, the present invention may contain: antimony (Sb) and tin(Sn), each of which has an effect of improving magnetic properties ofsteel, in the range of 0.0005% to 0.1%; boron (B), which has an effectof enhancing grain boundary strength of steel, in the range of 0.0005%to 0.01%; Ca and REM, each of which has an effect of controlling theform of oxide and sulfide and improving magnetic properties of steel, inthe range of 0.001% to 0.01%; Co and Ni, each of which has an effect ofimproving magnetic flux density of steel, in the range of 0.05% to 5%;and Cu, which is expected to provide strengthening by precipitation bymeans of aging precipitation, in the range of 0.2% to 4%, respectively.

The grounds for the limitations with regard to an embodiment of amanufacturing method according to the present invention will now bedescribed below.

In the present invention, the manufacturing process from steelmaking tocold rolling may be performed in accordance with methods commonly usedfor manufacturing general non-oriented electrical steel sheets. Forexample, steel, which was prepared by steelmaking and refined withpredetermined components in a converter or electric furnace, may besubjected to continuous casting or blooming after ingot casting toobtain steel slabs, which in turn may be subjected to process steps,including hot rolling, optional hot band annealing, cold rolling, finalannealing, insulating coating application and baking, and so on tomanufacture steel sheets. In these steps, the conditions for properlycontrolling the precipitation state will be described below. It shouldbe noted that hot band annealing may optionally be carried out after thehot rolling, and that the cold rolling may be performed once, or twiceor more with intermediate annealing performed therebetween.

The steel slabs composed of the aforementioned chemical compositions areto be subjected to hot rolling at a slab heating temperature of 1000° C.or higher to 1200° C. or lower. That is, if the slab heating temperatureis below 1000° C., it is not possible to achieve an effect of inhibitingthe growth of crystal grains during final annealing in a sufficientmanner due to the precipitation and growth of Ti carbides during slabheating. Alternatively, if the slab heating temperature is above 1200°C., this is not only disadvantageous in terms of cost, but also causesslab deformation due to a reduction in strength at high temperature,which interferes with, e.g., extraction of the steel slabs from theheating furnace, resulting in lower operability. Therefore, the slabheating temperature is to be within the range of 1000° C. to 1200° C.Additionally, the hot rolling itself is not limited to a particular typeand may be performed under the conditions of, for example, hot rollingfinishing temperature in the range of 700° C. to 950° C. and coilingtemperature of 750° C. or lower.

Then, the resulting hot rolled steel materials are subjected to optionalhot band annealing and cold rolling or warm rolling once, or twice ormore with intermediate annealing performed therebetween to be finishedto a final sheet thickness before final annealing. Prior to the finalannealing, it is important to subject the steel materials to heattreatment at least once where the steel materials are retained attemperatures of 800° C. or higher and 950° C. or lower for 30 seconds ormore. This heat treatment may allow precipitation of Ti carbides inmicrostructures prior to the final annealing and thereby inhibit thegrowth of crystal grains during final annealing.

That is, if the above-described heat treatment is performed attemperatures below 800° C., the resulting precipitation may beinsufficient, while above 950° C., the effect of inhibiting the growthof crystal grains during final annealing would be insufficient due tothe growth of precipitates.

Additionally, the aforementioned heat treatment is preferably performedin combination with either hot band annealing or intermediate annealingprior to the final annealing.

The subsequent final annealing may be performed at 700° C. or higher and850° C. or lower to thereby control the microstructure of recrystallizedgrains into a homogeneous and fine state, providing an electrical steelsheet having high strength and excellent magnetic properties. If thefinal annealing is performed at temperatures below 700° C., theresulting recrystallization is insufficient, while above 850° C.,crystal grains are prone to grow even when applying the presentinvention, resulting in a reduction in strength of the products.Following this final annealing, the steel materials are subjected toprocesses for applying and baking insulating coating thereon to obtainfinal products.

Example 1

Steel samples having compositions shown in Table 3 were obtained bysteelmaking in a vacuum melting furnace, heated to 1100° C., and thensubjected to hot rolling to be a thickness of 2.1 mm. Then, the sampleswere subjected to hot band annealing at 900° C. for 90 seconds andfurther to cold rolling to be finished to a thickness of 0.35 mm. Atthis moment, an evaluation was made of the occurrence of scab defects onthe surfaces of the steel sheets, using the scab size per unit area as areference. Subsequently, the samples were subjected to final annealingfor 30 seconds under two different conditions at 750° C. and 800° C.,respectively. Then, test specimens were cut parallel to the rollingdirection from the steel sheet samples thus obtained for tensile testand fatigue test. In addition, the magnetic properties were evaluatedbased on the iron loss with a magnetizing flux density of 1.0 T andfrequency of 400 Hz of the Epstein test specimens that were cut from thesamples in the rolling direction and transverse direction, respectively.The evaluation results are shown in Table 4.

TABLE 3 (mass %) Steel Si Mn Al P C N Ti Formula (1) Ti* Remarks 1 4.080.08 0.0010 0.012 0.0250 0.0015 0.0010 354 −0.0041 Comparative Example 24.10 0.05 0.0010 0.010 0.0247 0.0013 0.0189 354 0.0145 Inventive Example3 4.05 0.04 0.0004 0.018 0.0251 0.0016 0.0349 354 0.0295 InventiveExample 4 4.08 0.05 0.0015 0.011 0.0245 0.0012 0.0641 353 0.0600Comparative Example 5 4.02 0.04 0.0020 0.017 0.0258 0.0017 0.1164 3510.1106 Comparative Example 6 4.07 0.08 0.0019 0.014 0.0260 0.0019 0.1630354 0.1565 Comparative Example

TABLE 4 800° C. Annealing 750° C. Annealing Surface Fatigue Fatigue ScabTensile Limit Tensile Limit Defect Strength Strength Strength StrengthStrength Strength Rate W_(10/400) TS FS Ratio W_(10/400) TS FS RatioSteel (m/m²) (W/kg) (MPa) (MPa) FS/TS (W/kg) (MPa) (MPa) FS/TS Remarks 10.000 27.4 634 540 0.85 33.4 707 570 0.81 Comparative Example 2 0.00031.5 710 635 0.89 33.9 727 650 0.89 Inventive Example 3 0.005 33.7 715650 0.91 34.6 731 665 0.91 Inventive Example 4 0.159 42.7 722 600 0.8344.4 737 620 0.84 Comparative Example 5 0.189 46.5 726 560 0.77 48.3 744575 0.77 Comparative Example 6 0.211 48.0 734 565 0.77 51.0 750 580 0.77Comparative Example

It can be seen from Table 4 that Steel Sample No. 1, which has a Ti*value out of the scope of the present invention, exhibits significantlydifferent properties depending on the final annealing temperatures,which is considered problematic in terms of quality control. On theother hand, steel samples containing a proper amount of Ti show smallerdifference in their properties depending on the final annealingtemperatures, yielding high tensile strength in a stable manner.However, as compared to Steel Sample No. 2 and 3 having steelcompositions within the range specified by the present invention, SteelSample No. 4, 5 and 6, each having a Ti content out of the scope of thepresent invention, exhibit not so high fatigue limit strength for theirhigh tensile strength and have inferior scab rate and magneticproperties.

Example 2

Steel samples having compositions shown in Table 5 were obtained bysteelmaking in a vacuum melting furnace, heated to 1050° C., and thensubjected to hot rolling to be a thickness of 2.1 mm. Then, the sampleswere subjected to hot band annealing at 850° C. for 120 seconds andfurther to cold rolling to be finished to a thickness of 0.35 mm. Atthis moment, an evaluation was made of the occurrence of scab defects onthe surfaces of the steel sheets, using the scab size per unit area as areference. Subsequently, the steel samples were subjected to finalannealing at 800° C. for 30 seconds. Then, test specimens were cutparallel to the rolling direction from the steel sheet samples thusobtained for tensile test and fatigue test. In addition, the magneticproperties were evaluated based on the iron loss with a magnetizing fluxdensity of 1.0 T and frequency of 400 Hz of the Epstein test specimensthat were cut from the samples in the rolling direction and transversedirection, respectively. The results thereof are also shown in Table 6.

Additionally, Steel Sample No. 18, which does not satisfy the relationof formula (1) specified by the present invention, experienced sheetfracture during cold rolling, and so was not subjected to the subsequentevaluation process.

TABLE 5 (mass %) Steel Si Mn Al P C N Ti Others Formula (1) Ti* Remarks7 3.05 0.15 0.3500 0.018 0.0165 0.0014 0.0174 — 284 0.0126 ComparativeExample 8 3.75 0.08 0.0010 0.019 0.0043 0.0015 0.0172 — 329 0.0121Comparative Example 9 3.78 0.05 0.0008 0.014 0.0159 0.0017 0.0166 — 3290.0108 Inventive Example 10 4.01 0.04 0.0001 0.015 0.0135 0.0013 0.0154— 349 0.0109 Inventive Example 11 4.01 0.04 0.0004 0.015 0.0320 0.00160.0148 — 349 0.0093 Inventive Example 12 4.05 0.05 0.0004 0.013 0.05720.0016 0.0166 — 351 0.0111 Comparative Example 13 4.03 0.01 0.0004 0.0010.0175 0.0041 0.0168 — 343 0.0027 Comparative Example 14 4.82 0.041.0300 0.018 0.0158 0.0016 0.0188 — 419 0.0133 Inventive Example 15 3.020.88 0.7000 0.010 0.0289 0.0016 0.0333 — 317 0.0278 Inventive Example 163.55 0.59 1.2100 0.010 0.0294 0.0021 0.0328 — 344 0.0256 InventiveExample 17 4.30 0.11 0.1800 0.012 0.0285 0.0025 0.0322 — 380 0.0236Inventive Example 18 4.60 0.59 1.2100 0.010 0.0296 0.0011 0.0311 — 4540.0293 Comparative Example 19 4.03 0.15 0.0005 0.010 0.0144 0.00090.0244 Sb: 350 0.0213 Inventive 0.015 Example 20 4.11 0.08 0.0009 0.0110.0167 0.0021 0.0217 Sn: 356 0.0145 Inventive 0.043 Example 21 4.30 0.180.2530 0.007 0.0145 0.0009 0.0191 B: 0.003 382 0.0160 Inventive Example22 4.25 0.09 0.2310 0.018 0.0181 0.0011 0.0155 Ca: 381 0.0117 Inventive0.003 Example 23 4.22 0.15 0.0830 0.015 0.0226 0.0016 0.0185 REM: 3720.0130 Inventive 0.004 Example 24 3.98 0.25 0.2250 0.013 0.0284 0.00180.0355 Co: 0.25 358 0.0293 Inventive Example 25 4.05 0.20 0.2840 0.0160.0133 0.0015 0.0211 Ni: 0.15 367 0.0160 Inventive Example 26 3.87 0.180.2760 0.011 0.0336 0.0013 0.0347 Cu: 0.22 348 0.0302 Inventive Example

TABLE 6 Surface Scab Tensile Fatigue Defect Strength Limit Strength RateW_(10/400) TS Strength Ratio Steel (m/m²) (W/kg) (MPa) FS (MPa) FS/TSRemarks 7 0.001 28.6 625 510 0.82 Comparative Example 8 0.000 32.7 673535 0.79 Comparative Example 9 0.001 34.5 685 624 0.91 Inventive Example10 0.005 32.2 708 631 0.89 Inventive Example 11 0.005 31.2 705 650 0.92Inventive Example 12 0.230 38.7 694 575 0.93 Comparative Example 130.110 36.8 701 540 0.77 Comparative Example 14 0.005 28.8 779 715 0.92Inventive Example 15 0.035 34.5 668 607 0.91 Inventive Example 16 0.02633.3 703 645 0.92 Inventive Example 17 0.039 33.5 735 680 0.93 InventiveExample 18 — — — — — Comparative Example 19 0.003 31.9 701 620 0.88Inventive Example 20 0.004 31.2 707 635 0.90 Inventive Example 21 0.00633.4 733 640 0.87 Inventive Example 22 0.003 31.6 729 640 0.88 InventiveExample 23 0.003 32.0 721 633 0.88 Inventive Example 24 0.007 33.3 723645 0.89 Inventive Example 25 0.005 34.1 718 625 0.87 Inventive Example26 0.008 33.5 706 608 0.86 Inventive Example

It can be seen from Table 6 that each of the steel sheets according tothe present invention exhibits less scabs, good iron loss properties andhigh tensile strength, as well as high fatigue limit strength.

1. A non-oriented electrical steel sheet comprising, by mass %: Si: 5.0%or less; Mn: 2.0% or less; Al: 2.0% or less; and P: 0.05% or less, in arange satisfying formula (1), and the steel sheet further comprising, bymass %: C: 0.008% or more and 0.040% or less; N: 0.003% or less; and Ti:0.04% or less, in a range satisfying formula (2), the balance beingcomposed of Fe and incidental impurities:300≦85[Si %]+16[Mn %]+40[AI %]+490[P %]≦430  (1)0.008≦Ti*<1.2[C %]  (2), where Ti*=Ti−3.4[N %].
 2. The non-orientedelectrical steel sheet according to claim 1, wherein the Si, Mn, Al andP contents are, by mass %, Si: more than 3.5% but not more than 5.0%,Mn: 0.3% or less, Al: 0.1% or less, and P: 0.05% or less.
 3. Thenon-oriented electrical steel sheet according to claim 1, furthercomprising, by mass %, at least one of: Sb: 0.0005% or more and 0.1% orless; Sn: 0.0005% or more and 0.1% or less; B: 0.0005% or more and 0.01%or less; Ca: 0.001% or more and 0.01% or less; REM: 0.001% or more and0.01% or less; Co: 0.05% or more and 5% or less; Ni: 0.05% or more and5% or less; and Cu: 0.2% or more and 4% or less.
 4. A method formanufacturing a non-oriented electrical steel sheet, comprising:subjecting a steel slab to soaking, where the steel slab is retained ata soaking temperature of 1000° C. to 1200° C., the steel slabcontaining, by mass %, Si: 5.0% or less, Mn: 2.0% or less, Al: 2.0% orless, and P: 0.05% or less, in a range satisfying formula (1), and thesteel slab further containing, by mass %, C: 0.008% or more and 0.040%or less, N: 0.003% or less, and Ti: 0.04% or less, in a range satisfyingformula (2); subjecting the steel slab to subsequent hot rolling toobtain a hot-rolled steel material; then subjecting the steel materialto cold rolling or warm rolling once, or twice or more with intermediateannealing performed therebetween, to be finished to a final sheetthickness; and subjecting the steel material to final annealing, whereinprior to the final annealing, the steel material is subjected to heattreatment at least once where the steel material is retained attemperatures of 800° C. or higher and 950° C. or lower for 30 seconds ormore, and subsequently to the final annealing at 700° C. or higher and850° C. or lower:300≦85[Si %]+16[Mn %]+40[Al %]+490[P %]≦430  (1)0.008≦Ti*<1.2[C %]  (2), where Ti*=Ti−3.4[N %].
 5. The method formanufacturing a non-oriented electrical steel sheet according to claim4, wherein the Si, Mn, Al and P contents are, by mass %, Si: more than3.5% but not more than 5.0%, Mn: 0.3% or less, Al: 0.1% or less, and P:0.05% or less.
 6. The method for manufacturing a non-oriented electricalsteel sheet according to claim 4, wherein the steel slab furthercontains, by mass %, at least one of: Sb: 0.0005% or more and 0.1% orless, Sn: 0.0005% or more and 0.1% or less, B: 0.0005% or more and 0.01%or less, Ca: 0.001% or more and 0.01% or less, REM: 0.001% or more and0.01% or less, Co: 0.05% or more and 5% or less, Ni: 0.05% or more and5% or less, and Cu: 0.2% or more and 4% or less.